This is something I always wondered about: when I leave my charger plugged on the wall socket but not charging anything, is it still consuming energy?

Some notebook chargers have a led that is always on, so I guess it's consuming at least a bit. But what about Apple chargers? I want to leave my iPhone and iPad chargers always plugged under my desk (so I can just connect my device when I want to charge) but I'm worried they shouldn't be plugged all the time.

7 Answers
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I measured my MacBook Pro’s charger, and the iPhone/iPad chargers, with a Kill-a-Watt-like device, and it didn’t seem to use any significant power when plugged into the wall on its own (e.g. it was less than 1 watt).

BUT:

I’m not sure if my device is sensitive enough to measure very small amounts of power though.

your mileage may vary. If you’re interested in this sort of thing, I’d get a little dongle of your own and do some measuring. Mine cost about £5.

It does consume energy. Feel it, it will feel warm or even hot. It doesn't use much, but it is still a good idea to not waste this. Unplug it, or buy a power strip and turn off the power strip when it isn't needed.

To find out exactly how much, you could use a Kill-a-Watt, or read the MacBook Environmental Report (PDF). The current version (MacBook, May 2010, White) says the no-load power draw is 0.010 W. The older version (MacBook, 2008, Metal body) draws 0.15 W at no load. This is quite a difference.

0.15 W means if you left it plugged in for a year you would use 1.314 Kwh. Given a price of roughly $0.20/Kwh this would cost you about 25 cents per year. If the computer is on the whole time (at 14 Kwh) the cost would be about $25/year.

It will consume energy even when not connected to your Mac. Also, the longer you leave them plugged in, the more likely they will be damaged by an electrical surge or lightening strike etc. Best thing to do is probably have a multi-socket lead which your chargers plug into under your desk and you can just pull that out of the wall when not in use. That protects your equipment from electrical issues and saves your back from getting under the desk!

All your chargers are designed to support being plugged in 24 hours a day. Don't worry about them wearing out. If you are concerned about a small charge to your electric bill or if you're trying to do your part to waste less electricity, then you can plug them all into a power strip. When you aren't using them, turn off the power strip.

Your milage may vary ( and as others said, it does consume energy. Though it is less than 1 watt hour (calculator included) and it will probably cost you a few cents per year at most. At least the macbook adapter, which is probably consuming 0.1 kWh. Some mobile phones adapters may get to 0.5 kWh and even beyond. That makes a lot of difference and then it can cost up to 2 bucks per year. Calculating kWh at $0.2. Still an unnoticeable cost.

If you want to actually save money with small actions like that, I'll quote that great Michael link I've already gave it on the top:

Funny calculation: if you leave your macbook always plugged in, it will cost roughly those $25 per year another user also calculated. Now, if you unplug it every day for 9 hours you save about $10. But you'll be using battery energy, and charging it will be done at 80 kWh. It takes 2 hours to recharge 80%, so if you do this every day, it will still cost you the same $25. If you drain closer to 0 every day, it will take 4 hours to a full charge and cost you at least $37 total. If you just close the lid and use almost no power while unplugged, then the battery cost won't really kick in, and you can save those $10. Maybe it will be only $9.

(*) So it is quite ok to leave it on, but I wouldn't. Kind of.

We should always have at least a power strip for electronics and switch them down regularly anyway, for safety. At very least on a thunder storm, as hazards may happen. But high voltage discharges don't come only from neighbouring nature nor only from thunder. I just unplug almost all my devices from the wall.

Now the real point is: do you have an extra U$79 to spend and have handy an extra charger? If so, then just leave it on for convenience (and laziness)!

And if you want to be environmentally friendly, keep in mind those $1 a year multiplied by a over guess-timate of 200 million people who uses a macbook is still nothing ($5 million per year or 20 million kWh or 0.00000002 pentawatt-hour against 144 pentawatt-hour average global consumption estimate in 2008) and it's not like you can convince all those people to get off their convenience anyway.

Given the different circumstances, a charger for the iPhone 4/4s/5/5s/6 can be plugged in for 24/7 without a difference with charger. A minor current that is less than 0.01 percent of a watt goes into the basin of the charger and isn't enough to even be considered a worry. It'll equal out to a third of a cent a year.

I wouldn't recommend doing this. Also, never keep your devices plugged in constantly... This will reduce its battery life. It's a good idea to do a power cycle every month to keep a healthy battery (let the battery go down until it dies, then recharge fully).